RF data signals are generally either multi-level RF data signals or binary-level RF data signals. A communication system that communicates both multi-level RF data signals and binary-level RF data signals typically requires two RF data band pass filters; one for each type of signal. Two filters are required because multi-level RF data signals and binary-level RF data signals have substantially different filtering requirements, as described below. If two filters are not used, the system is either limited to communicating only one type of RF data signal or the system must employ a selectable RF data band pass filter. A selectable RF data band pass filter still limits the system to communicating only one type of RF data signal at a time and requires the filter to be altered whenever the other type of RF data signal is desired.
A typical multi-level RF data signal is an approximation of a sinusoidal waveform produced by quantizing, at fixed intervals, the sinusoid into multiple discrete currents or voltages. By varying the magnitude of the discrete currents or voltages, various tone frequencies can be obtained. The variation of the tone frequencies, at the fixed intervals, defines signalling formats, such as Select Five or Five Tone. To properly filter a multi-level RF data signal, the band pass filter must have a bandwidth of approximately 340 Hertz to 2900 Hertz. The attenuation at the first corner frequency (340 Hz) must be at least a two pole roll off, and the attenuation at the second corner frequency (2900 Hz) must be at least a three pole roll off. The multi-level RF data band pass filter must also have a tolerance of .+-.1 dB, with respect to a nominal level, over the entire bandwidth to minimize spectral distortion. In order to meet the above requirements, a multi-level RF data band pass filter generally has a group delay envelope in excess of 200 uSec. In part, because of the substantial group delay envelope, a multilevel RF data band pass filter will not properly filter binary-level RF data signals.
Typical binary-level RF data signals are fast frequency shift keying (FFSK) and Motorola's STAT ALERT.TM.. To properly filter a binary-level RF data signal, the band pass filter must have a group delay envelope of about 20 uSec when passing binary-level RF data signals in the frequency range of about 1100 Hertz to 1800 Hertz and about 35 uSec when passing binary-level RF data signals in the frequency range of about 950 Hertz to 1100 Hertz and about 1800 Hertz to 2000 Hertz. The group delay envelope requirements are tightly controlled such that the total harmonic distortion of the binary-level RF data signal is minimized. In addition, a binary-level band pass filter has a bandwidth requirement of approximately 900 Hertz to 2200 Hertz. The attenuation at the first corner frequency (900 Hz) and the second corner frequency (2200 Hz) are typically less than the multi-level band pass filter attenuation requirements. Further, the binary-level RF data band pass filter has a tolerance requirement of about .+-.1.45 dB, with respect to a nominal level, over the entire bandwidth. Because the bandwidth, attenuation, and tolerance requirements of a binary-level RF data band pass filter are substantially less than the requirements of a multi-level RF data band pass filter, the group delay envelope requirements of the binary-level RF data band pass filter are obtainable. As a result of the substantially different filtering requirements, prior art binary-level RF data band pass filters and prior art multi-level RF data band pass filters are not interchangeable.
A further difficulty of using one common band pass filter for both multi-level and binary-level RF signals is the strict international requirements for data communication signals. At present, the strictest international requirements are generated by the British Standard Institute. Under the British Standard guidelines the nominal level is -12 dBm, in comparison to the United States which requires the nominal be less than 10 dBm. The British Standard also requires that the energy levels of all signals having frequencies above 3600 Hertz be less than -34 dBm.
A need exists for radio frequency data communication band pass filter than will accommodate both multi-level RF data signals and binary-level RF data signals without selectable alterations, and will comply with international requirements.